Scintillators are materials that emit flashes or pulses of light when they interact with ionizing radiation. Scintillator crystals are widely used in radiation detectors for gamma-rays, X-rays, cosmic rays, and particles characterized by an energy level of greater than about 1 keV. It is possible to make radiation detectors, by coupling the crystal (or scintillator) with an element for detecting the light produced by the crystal when it interacts, or “scintillates,” when exposed to a source of radiation. The photo-detector produces an electrical signal proportional to the intensity of the scintillation (or light pulses received from the scintillator material). The electrical signal is then processed in various ways to provide data on the radiation.
Gamma-ray spectroscopy is an essential capability for radioactive isotope identification and is typically accomplished using inorganic scintillators or semiconductors. Recent advances have resulted in exceptional spectroscopic performance from both of these classes, yielding 662 keV energy resolution values of less than 3% for scintillator and less than 1% for semiconductors. However, high detector costs and low production yields remain as two significant shortcomings that prohibit their replacement of NaI(Tl) scintillators in large-scale applications.
Organic-based plastic and liquid scintillators loaded with high-atomic number elements (e.g. heavy metals) have been proposed and investigated as an alternate paradigm for NaI(Tl) replacement, owing to their very low cost and ability to scale to very large sizes. For example, 1-10% lead-loaded plastic scintillators are commercially available, although prior work has shown that there are several significant limitations that preclude their usefulness for gamma-ray spectroscopy: (1) major reduction in the scintillation light yield as the heavy-atom additive concentration is increased, (2) limited solubility and light transmission properties of the heavy-atom additives, and (3) poor photopeak sensitivity and energy resolution values for energies above 100 keV. These attributes are the consequence of relativistic heavy-atom quenching effects and strong absorptive losses from the metal complex. The latter limitation has been addressed through the cooperative use of an iridium compound and organometallic bismuth complex.
In summary, plastic compounds provide a low cost solution for large-volume scintillators. However, there have been significant drawbacks realized when loading such plastics with the heavier scintillating elements. Not the least of which is that heavy elements are known to quench both fluorescence and scintillation. Furthermore, simultaneously providing both gamma-ray spectroscopy and fast neutron discrimination properties to a plastic scintillator has heretofore eluded those of skill in the art.